This invention relates to an electrolytic process for the manufacture of ammonium tungstate, ammonium paratungstate, ammonium metatungstate and hydrated tungsten trioxide by the use of an anion exchange membrane, and more particularly relates to the manufacture of one of the above tungsten compounds by the control of the pH of the electrolyte in the anodic compartment.
An important and characteristic feature of the tungstates is the formation of condensed complex anions of isopolytungstates in acid solution. If one considers polytungstates as formed by the addition of hydrated acid (H.sub.3 O.sup.+) to the tungstate anion (WO.sub.4.sup.2.sup.-), then a series of isopolytungstates appear in which the ratio of hydrated acid groups to tungstate anion (and thus molecular weight), increases as pH is lowered. Paratungstates are formed at a ratio of 0.33 to 1.167; metatungstates are formed at a ratio of 1.5, and tungstic acid is formed at a ratio of 2.0.
These isopolytungstate compounds are widely used in the catalyst industry, usually as the ammonium salts, since the alkali metal and other salts would normally act as contaminants of the catalyst material.
Tungstic acid or hydrated tungsten trioxide is normally used as a starting material for the production of tungsten powder, which has a variety of uses including the starting material in various powder metallurgy processes.
Several techniques for the production of the ammonium tungstate compounds and tungstic acid exist.
Tungstic acid may be prepared by the acidification with mineral acid of most tungsten salts, for example sodium tungstate plus hydrochloric acid or calcium tungstate digested in hydrochloric acid gives a hydrated tungsten trioxide precipitate. This precipitate must then be washed, filtered and dried and must be kept strongly acidic during washing in order to prevent conversion to a colloidal form, rendering recovery by filtering exceedingly difficult. This process results in considerable free mineral acid and salt byproducts such as sodium chloride or calcium chloride, which must be disposed of.
Ammonium tungstate or ammonium paratungstate may be produced by dissolving hydrated tungstic trioxide in aqueous ammonia. This technique of course first requires the preparation of the hydrated tungsten trioxide, which tends to be time consuming and requires a large investment in equipment to handle the mineral acid.
Ammonium paratungstate may also be produced by the reaction in aqueous solution of ammonium chloride and sodium tungstate to precipitate insoluble ammonium paratungstate crystals. This method results in very low yields, and the ammonium paratungstate crystals tend to have entrapped sodium chloride, which cannot be easily removed.
Another method for the production of ammonium paratungstate is by liquid ion exchange (LIX) extraction of tungsten from sodium tungstate, followed by stripping the tungsten off the extractant with aqueous ammonia. This technique requires the addition of large amounts of strong mineral acid to the sodium tungstate solution in order to achieve the pH needed for LIX extraction. In consequence large amounts of the sodium salt such as sodium chloride or sodium sulfate are formed, which must be disposed of.
Ammonium metatungstate may be formed by dissolving hydrated tungsten trioxide in ammonium tungstate solution until a pH of about 2.5 to 4.0 is reached. This technique requires the addition of large amounts of hydrated tungsten trioxide which with lower pH values becomes less soluble in the solution. The excess must then be filtered out which may be difficult due to the tendency of the hydrated tungsten trioxide to colloidal formation. In addition, the yield is approximately 50%.
Another technique for the production of ammonium metatungstate is by addition of an ion exchange resin to ammonium tungstate solution or a slurry of ammonium paratungstate crystals until the required pH for formation of ammonium metatungstate is achieved. The use of ion exchange resins requires regeneration of these materials by the use of a strong mineral acid and the consequent formation of unwanted salt byproducts.
Other techniques for producing ammonium metatungstate involve boiling a slurry of ammonium paratungstate crystals or baking ammonium paratungstate crystals until at least about 6 moles of NH.sub.3 are lost. These techniques tend to require excessive times or excessive handling for the production of commercial quantities.
The search thus continues for simple and inexpensive methods for producing the ammonium isopolytungstates and tungstic acid in good yield and relatively pure form.